Scattering from turbid materials limits the depth through which images can be obtained. However, due to its deterministic nature, the scattering can be compensated with the help of a feedback mechanism using wavefront shaping. As a result, a focused spot can be created behind the scattering medium. Initial techniques have been limited by the inability to provide a feedback without access to the back side of the scatterer. Lately, new feedback mechanisms have been proposed such as iterative optimization feedback from fluorescence or digital OPC with second harmonic generation nanoparticles used as guide stars. These techniques are limited by a scarcity constraint that requires that the feedback signal only comes from a single particle to ensure single focus creation. However, this constraint can be overcome with iterative focusing methods by using a nonlinear feedback, such as two photon fluorescence, which itself is limited by optical power. Another promising method for imaging into scattering materials, especially biological tissue, uses a guide star created with an ultrasound focus. Ultrasonic waves propagate through soft tissue with three orders of magnitude less scattering than optical waves, allowing them to penetrate much deeper with minimal scattering. The ultrasound focus guide star locally modulates the frequency of light crossing it. These tagged photons are then used to record the scattered optical field, which when phase conjugated, delivers photons back to the ultrasound focus. Later, similar techniques were improved to allow for a reduction of the optical focus spot size.
A less explored feedback mechanism for focusing light through scattering materials is the photoacoustic effect. The photoacoustic effect produces acoustic waves as a medium absorbs light and undergoes thermal expansion. The photoacoustic effect is used in modern photoacoustic microscopy to image at depth in tissue. Photoacoustic microscopy differs from ultrasound imaging in that its contrast stems from optical absorption, as opposed to mechanical properties. Photoacoustics allows, for example, imaging of the vasculature by using hemoglobin in blood as the absorbing medium. Photoacoustic feedback has also been suggested for measuring the transmission matrix through a scattering material onto light-absorbing fibers. In this transmission matrix measurement the input optical modes were related to the absorbers found behind the scattering material. As a result, it was possible to localize particles along the axis of the transducer and create optical foci at the absorbers detected in the matrix. Unfortunately, none of these two early techniques has demonstrated so far imaging capability.